Invasive grass-fire cycles are instigating major state changes across western North American deserts. Invasion by the aggressive exotic annual cheatgrass, provides continuous fine fuel that increases the size, severity, and frequency of wildfires. Biotic resistance from native plant, consumers, and biological soil crusts are critical in preventing invasion. Fire reduces biotic resistance by removing native plants and biological soil crusts and altering rodent consumer behavior. However, as timing of precipitation events change from historical patterns, alterations in competitive plant relationships and individual plant responses could alter invasion outcomes. We collected 96, 6-inch diameter cores from an uninvaded area of the Great Basin desert fall of 2016. Each core had an assigned seed mix of individual invasive and native plants or a combination in a competitive mix. Each core was additionally assigned a fire treatment (burned, unburned) and a precipitation treatment (early—September 2016, late—October 2016) for a full-factorial replicated common garden in native environmental conditions. The fire treatment was applied by burning 15 g of seedless cheatgrass straw on top of each core in an aluminum cylinder. The following spring, the cores were harvested and the plants were sorted by species, measured, and weighed.
Results/Conclusions
Plant species had varying responses to the precipitation and fire treatments. Early precipitation increased cheatgrass and Sisymbrium altissimum densities compared with late precipitation timing. Cheatgrass densities were reduced more than 2-fold when grown in a competitive mix with other invasives, and more than 3-fold when grown with invasives and natives compared to when grown by itself. Fire did not have a large effect on cheatgrass densities. Together this may indicate that the loss of biotic resistance from native plant communities may be of greater importance than the loss of biological soil crusts. The native shrub Artemisia tridentata responded positively to fire and early precipitation when grown alone indicating that A. tridentata does have the potential to recover well after fire with reduction of direct plant competition. Competition with invasive and native species and early precipitation decreased density of Poa secunda compared with competition with just native species. When grown only with other natives, precipitation timing did not affect Poa secunda densities. As future climate models project earlier precipitation events this could mean a more rapid change from a native shrub and grass community to an exotic annual grassland as annual invasives overcome biotic resistance from native plants.